Drive assemblies are typically employed in power tools (e.g., electrically-operated power tools, pneumatic power tools, etc.) to transfer torque from a motor to a tool element to perform work on a workpiece. Particularly, impact wrenches utilize drive assemblies to convert continuous rotational motion of an output shaft of the motor to a striking rotational force, or intermittent applications of torque, to the tool element and workpiece. As such, impact wrenches are typically used to loosen or remove stuck fasteners (e.g., an automobile lug nut on an axle stud) that are otherwise not removable or very difficult to remove using hand tools. Such drive assemblies typically include a ram having at least one drive surface, and an anvil having at least one, typically flat driven surface oriented substantially normal to a longitudinal axis of the anvil.
The outer corner of the driven surface is typically rounded with a relatively small radius, providing a relatively sharp transition from the driven surface to an adjacent end surface of the anvil. With such a flat driven surface, imperfections in the form, size, and symmetry of the anvil may yield uneven contact between the ram and the anvil during operation of the impact wrench, potentially reducing the efficiency of the impact wrench and/or accelerating wear between the ram and the anvil.
Depending upon the size and configuration of the impact wrench, a relatively large amount of torque may be transferred through the drive assembly to the tool element and workpiece. As a result, relatively high contact stresses often occur at the outer corner of the driven surface during operation of the impact wrench.